Add Dockerfile-based Dokku deployment for monorepo layout

Uses a multi-stage Docker build that copies both app/ and blogex/,
preserving the path dependency. Includes release scripts, migration
module, and a sample Dokku setup script.

app/rel/overlays/bin/migrate

@@ -0,0 +1,6 @@
+#!/bin/sh
+set -eu
+
+cd -P -- "$(dirname -- "$0")"/..
+
+exec ./bin/firehose eval Firehose.Release.migrate